In complex analysis, a subfield of mathematics, a lacunary value or gap of a complex-valued function defined on a subset of the complex plane is a complex number which is not in the image of the function.
More specifically, given a subset X of the complex plane C and a function f : X → C, a complex number z is called a lacunary value of f if z ∉ image(f).
Note, for example, that 0 is the only lacunary value of the complex exponential function. The two Picard theorems limit the number of possible lacunary values of certain types of holomorphic functions.
- Clark, Douglas N., ed. (1999), Dictionary of Analysis, Calculus, and Differential Equations, Comprehensive dictionary of mathematics, 1, CRC Press, pp. 97–98, ISBN 9780849303203.